In integrated circuit processing, electromigration has been an unresolved problem. Electromigration is caused by a transfer of momentum between flowing electrons and stationary metal atoms. The flowing electrons strike the metal atoms in a lead and have the greatest effect where three or more metal grains form a common boundary point (called a triple point) therebetween. The smaller the grain size of the metal, the more chances there are for multiple triple points between grains in any given volume. After sufficient migration has occurred, the depleted regions are macroscopically observable as voids, often at the triple points, that eventually destroy electrical continuity.
Conventional integrated circuit leads are formed from a metallic substance such as an aluminum alloy. The leads are patterned using standard photoresist techniques and anisotropic etches to form approximately vertical sidewalls and a flat top surface. A structure thus formed is subject to greater stress near the sharp corners between the vertical sidewalls and the flat top surface. Stress can cause the leads to crack which further enhances the effects of electromigration and may cause a cascading of electromigration effects on the rest of the lead.
Previous attempts to prevent or reduce the effects of electromigration have focussed on controlling the metallurgical microstructures of the metal. Various heat treatments have been attempted to optimize the thermal treatment in a conventional furnace in order to enhance the grain size and orientation of the metal. In the previous attempts to reduce the effects of electromigration, the results have been unsatisfactory and the circuit fabrication process has been lengthened. Thus, there is a need for a method to greatly reduce the effects of electromigration in a semiconductor integrated circuit lead without adding an excessive amount of time to the fabrication process.